you are correct in that the ignition timing is locked at idle
what would affect the timing however would be any correction such as water, air, battery, boost etc
these all play a part and could drive the timing up or down, but this again, shouldn't be applied at idle
check out SETTING, ING/INJ which is the temp adjust, it shows you the realtime IGN value the FC is reading from the with correction applied
compare that to what you see at the CAS for any clues